The present invention relates to a reflex liquid crystal (electro-optical) display device adapted for a projection display system or the like, and also to a display apparatus, a projection optical system and a projection display system used in combination with such a display device.
With the recent progress in realizing improved projection display with a high definition, a small size and a high luminance, there are noted and practically utilized reflex display devices which are suited for achieving a dimensional reduction with an enhanced definition and are capable attaining a high optical efficiency.
Out of such display devices, there is reported an active reflex liquid crystal display device wherein a driving element is provided on a silicon substrate which is positioned opposite to a glass substrate having a transparent electrode formed therein and is composed of, e.g., a CMOS (complementary metal oxide semiconductor) circuit, and a driving circuit substrate having an aluminum optical reflecting electrode is placed thereon, and a vertically-aligned liquid crystal material is injected between the pair of such substrates (Paper (1): H. Kurogane et al., Digests of SID1998, p. 33–36 (1998); Paper (2): S. Uchiyama et al., Proceedings of IDW2000, p. 1183–1184 (2000)). The devices of this type have practically been commercialized by some makers.
Here, the vertically-aligned liquid crystal material is one having a negative permittivity anisotropy (i.e., Δε(=ε(∥)−ε(⊥), which is the difference between the parallel permittivity ε(∥) and the vertical permittivity ε(⊥) to the major axis of the liquid crystal molecule, is negative). When the voltage applied between its transparent electrode and light reflecting electrode is zero, the liquid crystal molecules are oriented to be substantially vertical to the substrate plane to thereby give display in a normally black mode.
The thickness (cell gap) of the vertically-aligned liquid crystal layer in the conventional reflex device reported in the above theses is 3 to 4 μm, and the curve of the liquid crystal transmissivity to the driving voltage applied to the liquid crystal (hereinafter referred to as V-T curve, which corresponds to the reflectivity of the device measured actually in the reflex device; it is supposed here that the incident light, e.g., s-polarized light, is modulated into p-polarized reflected light by the device as will be described later) has such characteristic that it rises at a threshold voltage of 2V or so and reaches its maximum value at an applied voltage of 4 to 6V. The transmissivity of the liquid crystal is changed analogously by changing the voltage between the electrodes to thereby realize expression of gradations. FIG. 14 graphically shows data excerpted as an example from Paper (1) cited above. According to the reported data, the liquid crystal layer has a thickness of 3 μm, the driving voltage is approximately ±4V, and the response speed (rise time+fall time) is 17 msec or so.
Normally the liquid crystal is driven while the voltage is inverted to be positive or negative per frame or field, so that the above device is actually driven by a voltage of ±4 to 6V at the maximum. (Since the positive and negative V-T curves are mutually symmetrical in principle, it is usual that the V-T curve is expressed as positive alone.) It is also defined that a liquid crystal driving voltage of ±4 to 6V needs to be more than 8 to 12V as an effective withstand voltage of a driving transistor.
Since this voltage is considerably higher than the withstand voltage in a normal MOS process, a high withstand voltage process for an LDD (lightly doped drain-source) structure or the like is applied to a liquid crystal driving transistor formed in each pixel on the silicon driving circuit substrate. Considering the production cost, power consumption and so forth, the withstand voltage is generally in a range of 8 to 12V. This is the reason that the known device is so designed as to have a V-T curve of ±4 to 6V at the maximum.
In the vertically-aligned liquid crystal material used in the known devices, the refractive index anisotropy Δn(=n(∥)−n(⊥), which is the difference between the refractive index n(∥) along the major axis of the liquid crystal molecule and the refractive index n(⊥) vertical thereto), has a value less than 0.1 (typically 0.08 or so), and the typical pixel pitch is 13.5 μm (pixel size 13 μm).
Recently, one defect of the liquid crystal display device concerning a low response speed thereof is attracting attention as a problem, and it is well known that raising the response speed is an important requisite. In general, the response speed (rise time and fall time) of the liquid crystal is proportional to the square of the thickness d of the liquid crystal layer, as expressed by Eq. (1) and Eq. (2) below. Therefore, reducing the thickness of the liquid crystal layer is effective to attain a higher response speed.
                              rise          ⁢                                          ⁢          time          ⁢                      :                    ⁢                                          ⁢          τ          ⁢                                          ⁢          on                =                              γ            ·                          d              2                                                          ɛ              ⁡                              (                0                )                                      ⁢                          Δ              ⁡                              (                                                      V                    2                                    -                                      Vc                    2                                                  )                                                                        (        1        )                                          fall          ⁢                                          ⁢          time          ⁢                      :                    ⁢                                          ⁢          τ          ⁢                                          ⁢          off                =                              γ            ·                          d              2                                            K            ·                          π              2                                                          (        2        )            (where γ: viscosity of liquid crystal, d: thickness of liquid crystal layer, Δε: permittivity anisotropy of liquid crystal, ε(0): space permittivity, K: elastic constant of liquid crystal, V: voltage applied to liquid crystal (liquid crystal driving voltage), Vc: threshold voltage)
However, in the vertically-aligned liquid crystal display device known heretofore, although the response speed thereof is rendered higher according to Eqs. (1) and (2) with reduction of the thickness of the liquid crystal layer, there arises another problem that the driving voltage required for saturating the transmissivity becomes higher. FIG. 15 graphically shows V-T curves obtained by reducing the thickness of the liquid crystal layer in a system using a liquid crystal material (where Δn=0.082) employed in a conventional device, and FIG. 16 graphically shows changes caused in the saturation voltage with the thickness d of the liquid crystal layer.
As shown in FIGS. 15 and 16, the saturation voltage of the device becomes sharply higher over 6V after the thickness d of the liquid crystal layer is reduced to 2.5 μm or less, and the saturation voltage reaches high as 10V or so when the thickness d is less than 2 μm. That is, the withstand voltage required for the driving transistor needs to be higher than 20V. In addition, when the thickness d is less than 1.5 μm, the absolute value of the transmissivity fails to reach 100%. In case the thickness d is 1 μm, the transmissivity attainable is merely 30% or so, while the threshold voltage is raised to be higher.
Such a phenomenon is considered to result from that, with a reduction of the thickness d (cell gap) in the vertically-aligned liquid crystal, the interaction exerted on the interface between the liquid crystal molecules and the orientation film becomes relatively greater to the directional change caused in the director of the liquid crystal molecules by the applied voltage. To the contrary, when the liquid crystal layer has a sufficient thickness, the director is rendered more mobile due to the property as a bulk, whereby the interaction on the interface is decreased in effect.
As described above, if the driving voltage becomes higher in the liquid crystal display device, proper driving fails to be performed in an ordinary driving device substrate of silicon. It is a matter of course that this problem can be solved by raising the withstand voltage of the pixel driving transistor, but generally the process is complicated with further disadvantages of increasing both the production cost and the power consumption. Further due to such a rise of the withstand voltage, it is unavoidable that the transistor size is enlarged. For this reason, it becomes extremely difficult to manufacture high withstand-voltage transistors in a small pixel size (or pitch) less than 10 μm or so in particular.
For the reason mentioned above, it is practically difficult, in any known reflex display device using the conventional vertically-aligned liquid crystal, to reduce the thickness of the liquid crystal layer to 2.5 μm or less.
Reducing the thickness of the liquid crystal layer as described slows down the rise (response speed) to the applied voltage and lowers the yield in manufacture of the device.
Further, in any projection optical system equipped with such known display device, the F number of the optical unit needs to be equal to or greater than 3.5 for maintaining a high contrast as will be explained below, hence bringing another problem that a high luminance is not attained.
In any projection system equipped with reflex liquid crystal display devices, as shown in FIG. 17, there is required an optical unit wherein luminous flux emitted from a lamp light source 1 is irradiated to reflex liquid crystal display devices 3R, 3G, 3B, each using vertically-aligned liquid crystal, via polarized beam splitters 2R, 2G, 2B which serve as polarized light separating devices for red (R), green (G) and blue (B) respectively, and the reflected light beams modulated by such devices are collected by a prism (X-cube prism) 4 which synthesizes the light beams of the individual colors, and then the composite light beam is projected as projection light 10(p) to an unshown screen via a projection lens 5.
Here, in an illumination optical unit for illuminating the reflex liquid crystal devices 3R, 3G, 3B, the white light (10(p ,s) composed of p-polarized component and s-polarized component) from the white lamp light source 1 is processed to be s-polarized light 10(s) via a fly-eye lens 6, a polarizer/converter 7, a condenser lens 8 and so forth. Subsequently the s-polarized light 10(s) is introduced to a dichroic color separation filter 9, and the light separated therethrough is sent to total reflection mirrors 11, 12 and a dichroic mirror 13 to consequently become light 10R(s), 10G(s) and 10B(s) of individual colors. Thereafter the light is incident upon the reflex liquid crystal display devices 3R, 3G, 3B respectively via the polarized beam splitters 2R, 2G, 2B, and the reflected light is polarized and modulated in accordance with the voltage applied to the reflex liquid crystal display devices 3R, 3G, 3B. After incidence upon the polarized beam splitters 2R, 2G, 2B again, only the p-polarized components 10R(p), 10G(p), 10B(p) of the light are transmitted and then are synthesized by the prism 4. Consequently, when the applied voltage is zero in the reflex liquid crystal display device, the incident light is reflected directly as s-polarized light without passing through the polarized beam splitter, and thus the system is placed in a normally black mode where the light is polarized and modulated with a rise of the applied voltage, so that the p-polarized reflected light is increased to eventually raise the transmissivity (refer to FIG. 14).
In the optical unit employed for the known vertically-aligned liquid crystal display device reported in Papers (1) and (2), the F number is equal to or greater than 3.5 (e.g., 3.8 to 4.8 in Paper (1), or 3.5 in Paper (2)). The F number of the optical unit is a function of the incidence angle (outgoing angle of reflected light) θ of the light incident upon the device, and it is expressed as follows.F=1/(2×sin θ)  (3)
An expression of F=3.5 signifies that the device face is illuminated by the light within an angle of θ=±8.2° centering around a line perpendicular to the device face, and the reflected light is obtained therefrom.
As obvious from Eq. (3), the smaller the F number, the light incidence and outgoing angle θ become greater to consequently increase the total luminous flux, hence raising the luminance. However, in the reflex liquid crystal device, generally the black level value (transmissivity in a black state) becomes higher with an increase of the incidence angle, and the polarized-light separation characteristic of the polarized beam splitter is dependent on the angle θ, whereby it is unavoidable that the characteristic is deteriorated with an increase of the angle θ, and the degree of separation into the p-polarized light component and the s-polarized light component is rendered lower when the angular component is great. For the reasons mentioned, there occurs a phenomenon that the black level rises while the contrast is considerably lowered.
Thus, in practical use, there exists a problem of trade-off (difficulty for compatibility) between the luminance and the contrast. Because of this problem, in any conventional projection system equipped with such a known device, there is employed an optical unit where the F number is greater than 3.5 (more specifically, the F number of the projection lens 5 or that of the illumination optical unit). That is, in any projection optical system equipped with the known device, the F number is not settable to less than 3.5 due to a demand for practically realizing a high contrast to a certain degree, hence causing a failure in raising the luminance.
It is therefore a first object of the present invention to provide improvements in a vertically-aligned liquid crystal display device which is represented by a reflex liquid crystal display device of the invention having a high response speed, wherein the liquid crystal transmissivity reaches saturation at a low voltage despite a small thickness of the liquid crystal layer, and the device can be driven with facility on a driving circuit substrate manufacturable by an ordinary withstand voltage process even in a small pixel size. The above improvements also connote a display apparatus, a projection optical system and a projection display system using such a reflex liquid crystal display device of the invention.
A second object of the present invention resides in providing a projection optical system and a projection display system where a sufficiently low black level can be maintained in addition to the above accomplishment even in a high-luminance optical unit having a small F number, hence achieving a practically high contrast (i.e., meeting the requirements for both a higher luminance and a higher contrast in comparison with those of any conventional system).